add xml decimal attribute escaper

[happyhua]

Extra xml decimal attribute escaper is added in this PR.

It works similar as the existing xml attribute escaper. The main difference is that it will normalize a character as line feed \n in 
 instead of 


This can be useful if you to want to have the same print result as javax.xml.transform.Transformer.

Also a browser like google chrome will print 
 for a line feed \n character when calling XMLSerializer.serializeToString.

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